Natural Gas Properties with ISO6976.2016

In ISO6976.2016: Calorific Values and Properties of Natural Gas per ISO 6976:2016

knitr::opts_chunk$set(
  collapse = TRUE,
  comment  = "#>"
)
library(ISO6976.2016)

Overview

ISO6976.2016 implements the calculation method of ISO 6976:2016 "Natural Gas — Calculation of calorific values, density, relative density and Wobbe indices from composition". Given a gas composition as mole fractions, it returns all combustion and volumetric properties together with their standard uncertainties, propagated according to Annex B of the standard.

Properties returned

| Symbol | Description | Unit | |--------|-------------|------| | M | Molar mass | kg/kmol | | Z | Compression factor | — | | G_o, D_o | Ideal-gas relative density and density | —, kg/m³ | | G, u_G | Real-gas relative density | — | | D, u_D | Real-gas density | kg/m³ | | Hcg, u_Hcg | Molar gross calorific value | kJ/mol | | Hcn, u_Hcn | Molar net calorific value | kJ/mol | | Hmg, u_Hmg | Mass-basis gross calorific value | MJ/kg | | Hmn, u_Hmn | Mass-basis net calorific value | MJ/kg | | Hvg, u_Hvg | Volumetric gross calorific value (real gas) | MJ/m³ | | Hvn, u_Hvn | Volumetric net calorific value (real gas) | MJ/m³ | | Hvg_o, Hvn_o | Ideal-gas volumetric calorific values | MJ/m³ | | Wg, u_Wg | Gross Wobbe index | MJ/m³ | | Wn, u_Wn | Net Wobbe index | MJ/m³ | | Wg_o, Wn_o | Ideal-gas Wobbe indices | MJ/m³ |

Properties with a u_ prefix are standard uncertainties (k = 1) unless the coverage argument is set to a value other than 1.

Application limits

The calculation is valid only within the limits defined in ISO 6976:2016 §5:

• Combustion temperature (combustionTemperature): 0, 15, 15.55, 20, or 25 °C
• Volume reference temperature (volumeTemperature): 0, 15, 15.55, or 20 °C
• Reference pressure (pressure): 90–110 kPa (default 101.325 kPa)
• The compression factor Z must exceed 0.9

Quick start

The simplest way to provide a composition is to construct a numeric vector of length 60 in the component order of ISO 6976:2016 Table A.2 (use componentNames() to see the full list):

x   <- numeric(60)   # all zeros
u_x <- numeric(60)   # all zero uncertainties
r_x <- diag(60)      # identity correlation matrix (no correlations)

# Fill in a simple two-component mixture: 95 % methane, 5 % nitrogen
x[componentIndex("methane")]  <- 0.95
x[componentIndex("nitrogen")] <- 0.05

# Assign standard uncertainties (0.05 mol/mol each)
u_x[componentIndex("methane")]  <- 0.0005
u_x[componentIndex("nitrogen")] <- 0.0005

res <- calculateProperties(x, u_x, r_x,
                           combustionTemperature = 25,
                           volumeTemperature     = 15)

cat("Molar mass              :", round(res$M,   4), "kg/kmol\n")
cat("Compression factor Z    :", round(res$Z,   6), "\n")
cat("Gross CV (volumetric)   :", round(res$Hvg, 4), "MJ/m³\n")
cat("  standard uncertainty  :", round(res$u_Hvg, 6), "MJ/m³\n")
cat("Gross Wobbe index       :", round(res$Wg,  4), "MJ/m³\n")
cat("  standard uncertainty  :", round(res$u_Wg, 6), "MJ/m³\n")

Using the GasComponents class

For applications that build a composition incrementally — for example when reading chromatograph results component by component — the GasComponents R6 class provides named getters and setters:

gc <- GasComponents$new()

# Set fractions by name or by index
gc$setFraction("methane",  0.9234)
gc$setFraction("ethane",   0.0254)
gc$setFraction("propane",  0.0152)
gc$setFraction("nitrogen", 0.0103)
gc$setFraction("carbon dioxide", 0.0154)
gc$setFraction(1L, 0.9234)   # same as "methane"

# Set uncertainties
gc$setUncertainty("methane",        0.000332)
gc$setUncertainty("ethane",         0.000243)
gc$setUncertainty("propane",        0.000148)
gc$setUncertainty("nitrogen",       0.000195)
gc$setUncertainty("carbon dioxide", 0.000111)

# Retrieve a value
gc$getFraction("methane")

# Pass directly to calculateProperties
res <- calculateProperties(gc$fractions, gc$uncertainties, gc$correlations,
                           combustionTemperature = 15,
                           volumeTemperature     = 15)
round(res$Hvg, 4)   # real-gas vol. gross CV [MJ/m³]

Setting correlations

When a GC calibration provides a full covariance matrix, use setCorrelationMatrix() or setCorrelation() for individual pairs:

gc2 <- GasComponents$new()
gc2$setFractionArray(gc$fractions)
gc2$setUncertaintyArray(gc$uncertainties)

# Negative correlation between methane and ethane (typical for GC)
gc2$setCorrelation("methane", "ethane", -0.65)
gc2$getCorrelation("methane", "ethane")
gc2$getCorrelation("ethane", "methane")   # automatically symmetric

Reference conditions

ISO 6976:2016 defines properties at specified reference temperatures and a reference pressure. The package supports all temperature combinations in the standard:

data("example3", envir = environment())

# German/European standard: 25 °C combustion, 0 °C metering
r25_0 <- calculateProperties(example3$fractionArray, example3$uncertaintyArray,
                             example3$correlationMatrix,
                             combustionTemperature = 25,
                             volumeTemperature     = 0)

# UK/legacy standard: 15 °C / 15 °C
r15_15 <- calculateProperties(example3$fractionArray, example3$uncertaintyArray,
                              example3$correlationMatrix,
                              combustionTemperature = 15,
                              volumeTemperature     = 15)

cat("Hvg at 25/0 °C  :", round(r25_0$Hvg,  5), "MJ/m³\n")
cat("Hvg at 15/15 °C :", round(r15_15$Hvg, 5), "MJ/m³\n")

Uncertainty and coverage factor

All u_ outputs are standard uncertainties (k = 1) by default. For expanded uncertainties at a given confidence level, set coverage = 2 (approximately 95 % for a normal distribution):

data("example1", envir = environment())

r_k1 <- calculateProperties(example1$fractionArray, example1$uncertaintyArray,
                             example1$correlationMatrix,
                             combustionTemperature = 15, volumeTemperature = 15,
                             coverage = 1)
r_k2 <- calculateProperties(example1$fractionArray, example1$uncertaintyArray,
                             example1$correlationMatrix,
                             combustionTemperature = 15, volumeTemperature = 15,
                             coverage = 2)

cat("Gross Wobbe index :", round(r_k1$Wg, 5), "MJ/m³\n")
cat("  u (k=1)         :", round(r_k1$u_Wg, 6), "MJ/m³\n")
cat("  U (k=2, ~95%)   :", round(r_k2$u_Wg, 6), "MJ/m³\n")

Verification against ISO 6976:2016 Annex D

The package ships with the four reference datasets from Annex D of the standard. The results below reproduce Table D.2 of ISO 6976:2016.

data("example1", envir = environment())
res <- calculateProperties(example1$fractionArray, example1$uncertaintyArray,
                           example1$correlationMatrix,
                           combustionTemperature = 15,
                           volumeTemperature     = 15,
                           coverage              = 1)

tab <- data.frame(
  Property = c("M [kg/kmol]", "Z", "Hcg [kJ/mol]", "u(Hcg)",
               "Hmg [MJ/kg]", "u(Hmg)", "Hvg [MJ/m\u00b3]", "u(Hvg)"),
  Computed = round(c(res$M,     res$Z,     res$Hcg,   res$u_Hcg,
                     res$Hmg,   res$u_Hmg, res$Hvg,   res$u_Hvg), 7),
  ISO_6976 = c(17.3884301, 0.99776224, 906.1799588, 0.615609872,
               52.113961,  0.024301,   38.410611,   0.026267)
)
knitr::kable(tab, align = "lrr")

Component reference

# All 60 components in table order
nms <- componentNames()
cat(paste(sprintf("%2d  %s", seq_along(nms), nms), collapse = "\n"), "\n")

Further reading

• ISO 6976:2016 — the primary reference for all formulas, constants, and application limits.
• ?calculateProperties — full parameter documentation and output table.
• ?GasComponents — R6 class reference.
• ?example1 through ?example3_ex — dataset documentation.

ISO6976.2016 documentation built on April 9, 2026, 5:09 p.m.